1. Field of the Invention
The present invention relates to a heating time control apparatus and a method thereof for a microwave oven, and particularly to an improved heating time control apparatus and a method for a microwave oven capable of detecting food temperature using art infrared ray coming from food being heated, whereby better heating time control can be possible.
2. Description of the Conventional Art
Conventionally, when cooking food using a microwave oven, various kinds of sensors are used for judging a cooking state of food. Among them, a temperature sensor, a moisture sensor, or a gas sensor are generally used.
A temperature sensor is intended to detect a predetermined variation of a resistance value in accordance with a temperature variation and converts the detected variation into an electric signal. A moisture sensor is intended to detect a predetermined variation of a resistance value in accordance with a density variation of vapor in a heating compartment and converts the detected variation into an electric signal, at this time the moisture is in the form of absolute moisture. In addition, a gas sensor is intended to detect a predetermined variation of a resistance value in accordance with a gas density coming from food being cooked and converts the detected variation into an electric signal.
However, there are some disadvantages in correctly detecting a food cooking state when cooking food with a low heat. Referring to FIG. 1, the variations of the temperature, moisture and gas which are detected by such sensors are shown therein. The control of heating time is performed with a basis of assuming that a point where an output signal is sharply increased is a boiling point (BP) in case of water. However, in case of warming food and unfreezing a frozen meant or fish, or in case of cooking just after a food is cooked, a point where a predetermined output signal is varied is different from a boiling point shown in FIG. 1. Therefore, it becomes difficult to correctly control a total heating time with a basis of such as boiling point.
In an attempt to resolve such disadvantages, there is introduced one method of using a predetermined infrared ray using a pyroelectric effect so as to detect a surface temperature of food. Here, the pyroelectric effect is defined as forming a predetermined electric charge around a predetermined object which is caused as an object which is in an electrically neutralized state is heated thereby breaking a balanced state thereof whereby it is combined with floated ions having opposite electric charges from each other in the air. In addition, the method of using an infrared sensor using the pyroelectric effect for controlling a heating time is directed to detect the surface temperature of food by detecting a variation of electric charge around food varied in accordance with the amount of infrared ray which is varied in accordance with the temperature of food.
Such a method described above has better performance in detecting the surface temperature of food than using a moisture sensor, gas sensor or temperature sensor because the moisture, gas and temperature sensors is intended not to detect the temperature of food but to detect and control the moisture, gas and temperature in the heating compartment. Therefore, such sensors have disadvantages in detecting food temperature rather than using the pyroelectric effect method.
Even though the pyroelectric effect method has better performance in detecting food temperature, there are still some problems in that detecting food temperature because it is intended to use a differential type sensor for correcting and outputting a difference between a heating compartment temperature and a food temperature, so that when a temperature difference occurs slowly it is difficult to correctly detecting food temperature. In addition, to adapt such a method, there should be provided a chopper mounted in front of an infrared sensor so as to forcibly make a predetermined temperature variation therein, whereby the manufacturing cost increase.